The light weight and strength of magnesium and magnesium alloys makes products fashioned therefrom highly desirable for use in manufacturing critical components of, for example, high performance aircraft, land vehicles and electronic devices.
One of the most significant disadvantages of magnesium and magnesium alloys is corrosion. Exposure to the elements causes magnesium and magnesium alloy surfaces to corrode quickly, corrosion that is both unesthetic and reduces strength.
One strategy used to improve corrosion resistance of metal surfaces is painting. As the surface is protected from contact with corrosive agents, corrosion is prevented. However, many types of paint do not bind well to magnesium and magnesium alloy surfaces.
Methods based on chemical oxidation of an outer metal layer using chromate-solutions are known in the art as useful for treating magnesium and magnesium alloy surfaces to increase paint adhesion, see for example U.S. Pat. No. 2,035,380 or U.S. Pat. No. 3,457,124. However the low corrosion-resistance of treated surfaces and environmental unfriendliness of chromate solutions are definite disadvantages of these methods.
In PCT publication WO 99/02759 is described a method of providing a protective coating to a magnesium surface by polymerizing an electrostatically deposited resin comprising a variety of functional groups.
Several methods of metal surface treatment using silane solutions have been disclosed, see for example U.S. Pat. No. 5,292,549, U.S. Pat. No. 5,750,197, U.S. Pat. No. 5,759,629 and U.S. Pat. No. 6,106,901. Silane solutions are environmentally friendly and lend excellent corrosion resistance to treated metal surfaces. Silane residues from the solution bind to a treated metal surface preventing oxidation and forming a layer to which commonly-used polymers, such as paint, adhese, see U.S. Pat. No. 5,750,197. Although successfully applied to steel, aluminum, zinc and the respective alloys, magnesium and magnesium alloys have not been successfully treated with silane solutions.
U.S. Pat. No. 5,433,976 teaches alkaline solutions for the treatment of metal surfaces, the solutions including an inorganic silicate, inorganic aluminate, a cross-linking agent, and a silane. However, U.S. Pat. No. 5,433,976 does not teach the use of this solution for treating magnesium.
Another strategy used to improve corrosion resistance of metal surfaces is anodization, see for example U.S. Pat. No. 4,978,432, U.S. Pat. No. 4,978,432 and U.S. Pat. No. 5,264,113. In anodization, a metal surface is electrochemically oxidized to form a protective layer. Although anodization of magnesium and magnesium alloys affords protection against corrosion, adhesion of paint to anodized magnesium surfaces is not sufficient. Further, as discussed in U.S. Pat. No. 5,683,522, anodization often fails to form a protective layer on the entire surface of a complex workpiece.
It would be highly advantageous to have a method for treating magnesium or magnesium alloy surfaces so as to increase corrosion resistance beyond what is known in the art.